Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Optical systems suffer loss due to various forms of optical aberration, which lead to loss in signal information. In smaller, simpler optical devices, beams can be propagated along trajectories closely parallel to the optical axis. In these “paraxial” configurations, aberrations are small and can generally be ignored in practice. However, as more complex devices are built to perform advanced functions, the need to propagate beams off-axis and outside the paraxial region is becoming increasingly important. In these “higher order optics” situations, a number of monochromatic optical aberrations become more distinct. In particular, off-axis curvature of the focal plane of optical elements becomes a concern. So too does spherical aberration.
Specifically, in the field of optical add/drop multiplexers and switches, devices are being developed with higher numbers of input and output ports. These ports are disposed in linear arrays that extend transversely across the optical axis. Therefore, with higher port count devices, fibers extend further from the optical axis and switching optical beams to those fibers means that the effects of optical aberrations become greater.
Another issue contributing to aberrations is the size and profile of an optical beam. In switching devices it is often advantageous to reshape the beam profile to be highly asymmetric. For example, in liquid crystal on silicon (LCOS) based switches, elongate beam profiles are advantageous for efficiently switching many wavelength channels simultaneously. Larger and more asymmetric beams generally experience higher aberrations than smaller symmetric beams.
Further, in some optical switching devices, it is advantageous for individual wavelengths and polarization states to be spatially separated and propagated independently. In these cases, monochromatic aberrations sometimes lead to various forms of optical loss, including wavelength dependent loss, polarization dependent loss (PDL) and port dependent loss. These effects are undesirable from a performance point of view.
Attempts have been made to individually address the resulting losses incurred in these systems. For example, U.S. Pat. No. 6,813,080 entitled “Metal-free gratings for wavelength-multiplexed optical communications” to Raguin and Marciante (Assigned to Corning Incorporated) discloses a diffraction grating formed of layers of two types of silicon based material having different refractive indices to reduce PDL. A grism utilizing such a grating and being formed primarily of silicon is also proposed. By forming the diffraction grating from particular material layers, U.S. Pat. No. 6,813,080 seeks to reduce the PDL that the dispersive device itself introduces.
However, to the inventors' knowledge, no suitable techniques have been developed to actively address the above-mentioned aberrations at a system level, particularly in higher port count optical devices and devices incorporating asymmetric beams profiles. In particular, the device of U.S. Pat. No. 6,813,080 is not suitable for reducing loss in an optical system other than the PDL specifically introduced by an alternative grating/grism.
As an example of optical loss resulting from aberration, PDL is the relative attenuation experienced between constituent polarization components of an optical signal in propagation through an optical device or system. One specific definition of PDL is the peak to peak difference in transmission of an optical signal relative to all possible polarization states, after propagating through a device or system. That is:
      P    ⁢                  ⁢    D    ⁢                  ⁢    L    ⁢                  ⁢          (              ⅆ        B            )        =      10    ⁢                  ⁢    log    ⁢                  ⁢                  log        10            ⁡              (                              P            max                                P            min                          )            
PDL is wavelength dependent and is particularly prominent at high data rate transmission. Unlike other types of optical loss, PDL cannot be easily compensated for by simple amplification. PDL is enhanced in optical systems where polarization states are required to be spatially separated for polarization diversity purposes. Such systems include those that possess polarization dependent components, including liquid crystal elements. Many commonly used wavelength selective switch (WSS) devices fall into this category.
Therefore, as optical systems emerge that can operate at ultra-high data rates, and across large numbers of wavelength channels, it is becoming increasingly important to efficiently manage aberrations that give rise to loss such as PDL.